London to Paris and Paris trip help!

siriravi, your questions are much too broad for anyone here to be able to answer.

Based on just how little you seen to know about where you're going, I would say yes, you should go with a packaged tour. Otherwise, you need to do the same research other independent travelers do to figure out what to see, where to stay and how to get there. If your trip is in just a month or so, you would need to get some guidebooks and figure that out.

But going back to your questions, what is "reasonable" transportation cost for your family from London to Paris? What do you want to spend? I would say at this late date that Eurostar is probably your most efficient choice but maybe not.

I will ask why you are staying close to Heathrow if you are sightseeing London? That, to me, is not a economical or efficient choice.

Most economical choice in Paris? What do you want to spend per night? My "economical" might not be yours but I will say that you do not want to be on the outskirts.

What do you want to see? What are you interested in?

You mean within five to seven weeks? At this late date you have missed the opportunity to take advantage of discount tickets on the Eurostar or other transportation. Ask the same question -- why stay near Heathrow? It is about hour into the London via the tube from Heathrow.

Eurostar would be the easiest option if you were staying in London because the train is basically city center to city center. But since you are staying next to Heathrow you might consider flying. However, most of the discount airlines fly out of Stanfield that is on the other side of London. You can get to St Pancras station via tube in about hour and half with one change (I think) from near Heathrow. A taxi may run 75 to 100 pounds.

As for Paris use Steves' guide or use Trip Advisor for a hotel. We recently used Etoile Saint Honore (hotel-paris-honore.com) Just off Champs Elysees and within sight of the Arch. One small advantage but not to be overlooked. They give you a map of sister hotels and a business card. Show the business card and you can use the restroom in any of the other hotels. Handy.

Any good guidebook will list the must see sites. Go to your public library and check out guidebooks and travel DVDs to give a preview of what to see and do. Time is short so go quick.

To help tighten up Frank's post, the flights go from Stansted. The Piccadilly tube line runs direct from Heathrow to Kings Cross/St Pancras in about 55 minutes and all you have to do is follow the signs to the Eurostar.

What do you mean by "lodging close to Heathrow"? Where?

Have you done any planning and reading yet or are you just starting out? Do you have to return to London to get home or are you flying out of Paris? To where?

Should we consider going with a tour company or plan by ourselves?

If, as it seems, you have not done any planning for the trip yet, you probably should see a travel agent or 'tour company' to handle the planning. .

"2 kids" is that something like ages 2 and 4 or 15 and 17? Makes a difference for lodging and in some circumstances transportation costs.

Which places are the most recommended to visit?

What are your interests?

Are you staying at Heathrow for a reason? Or does it just seem like the place to stay. Rather than stay by Heathrow might I suggest Earl's Court area. Straight shot on the Tube to and from Heathrow and if you are going to take the Eurostar to Paris then that is easier and shorter too.

siriravi, Why are you staying near Heathrow? And how "near"? The Piccadilly line runs from Heathrow into central London, but depending where you are exactly other options may exist. London has a lot of transport.

I suggest you do lots of internet searches "what to see in London" and "what to see in Paris", then you can see what your family likes.

How long are you staying in London?

Please reply and answer our questions, then we can help you more.

Here is a basic tool for visiting London: https://tfl.gov.uk/
It covers government mass transit services and has a planner so you can plot how to get from here to there.
Google Maps has a similar trip planner and it will also show hotels, pubs etc. around a specific location. Your link to Google Maps may vary according to where you are based.
International hotel reservations services on-line such as Hotels.com and Booking.com include mapping and other tips. In both London and Paris you can find independent local versions of these on-line agencies.
There are a huge number of sites for Paris planning, so crank up Google. For rapid transit, https://www.ratp.fr/en
Regarding Eurostar, it is the most comfortable and probably fastest from downtown London to the centre of Paris. Cheapest: Probably bus connections. Planes are fastest, when in the air, but the surrounding procedures are time-consuming and stressful. As has been noted, you have missed the cheapest fares on both Eurostar and airlines.
You should consider carrying a smart phone, tablet, etc. that can connect to the internet for day-to-day research while on the road.
Have you read the destinations advice right here on this website (top left button, Explore Europe)? Many of your questions have already been addressed there. Other travel advice companies maintain similar guides.

Why stay near Heathrow? Because we will be staying with my sister who lives there.

Do you have to return to London to get home?
We will be flying from U.S. into London on March 24 morning and flying out from London on April 1,morning flight.

My kids ages are 10,13

Have you done any planning?
Planning to visit London half day on 24,25,26,half day on 27th
Travel to Paris on 27th night and stay in Paris for 2-3 nights.
Places of interest in Paris and am aware of are:
Eiffel Tower
Notre-Dame Cathedral
Sainte-Chapelle
Louvre Museum
Champs Elysees
Arc de triomphe
Versailles Palace (heard it takes a day to visit this place and have to book a tour to visit, any suggestions for planning purposes)

Versailles Palace (heard it takes a day to visit this place and have to book a tour to visit, any suggestions for planning purposes)

Not true. You can just buy train tickets and take the frequent RER C trains to Versailles Rive Gauche-Chateau. If you have a Museum Pass, just go through the security line to enter. This will eat up the better part of a day, and with all the other things you have on your list, and really only 2 full days to do them, think about skipping it.

I think you should fly from Heathrow to Paris. I think you can get tickets at less than 100 GBP per person.

You can still get Eurostar tickets for reasonable prices. Just got a quote for $457 round trip on your dates (3/27-3/30) for your family make-up. But got to buy soon. Not convenient for departure, going to St Pancras, but convenient arrival in center of Paris.
https://www.eurostar.com/us-en

As far as lodging, the Citadines chain as several places in Paris that are like apartments with kitchenettes. The ones close to the center, where you can walk to a lot of your sights, are more expensive than ones further away where you'll need to use the Metro to get to where you want to go.

If you are staying near Heathrow, I'd still go by Eurostar, taking the Piccadilly Line from Heathrow or whatever station is nearest where you are staying to Kings Cross. If you go by air, you'll have to deal with a train journey into town from the airport while tired from the trip and using a foreign language, while by Eurostar the Underground trip will be when you are fresh and everything will be in English.

Travel to Paris on 27th night and stay in Paris for 2-3 nights.

Ravi, two nights PLUS travel time to get to Paris will only leave you 1 full day to explore the city: not enough time for all the things on your list, and definitely not enough for Versailles. I wouldn't do any less than 3 nights and I still don't consider that time enough for Versailles.

If you want to go up in the Eiffel, make advance reservations:
https://www.toureiffel.paris/en

For Sainte-Chapelle, Louvre and Arc de Triomphe, you might consider a 2-day Paris Museum Pass. 2 FULL Days/3nights would probably allow you to slip in a couple other attractions in the city that the pass covers, such as the Towers at Notre Dame, Conciergerie, Cluny (we really like that one) or d'Orsay. But do consider what is interesting to you? There's no need to spend a lot of time in art museums if you and your family do not enjoy them. You can also visit the Arc de Triomphe at night to free up some time during the day. Personally? I'd skip the Champs: one of the least interesting streets in Paris.

http://en.parismuseumpass.com/rub-t-presentation-and-advantages-2.htm

Do note that the Louvre is closed on Tuesday (your arrival day).